Active screen plasma nitriding of Al using an iron cage: Characterization and evaluation

• Al samples were plasma nitrided in an iron active screen.
• At long nitriding periods, an AlN layer was formed under iron nitride coating.
• Contrary to the corrosion resistance, the mechanical properties of Al were improved.
• Iron nitride coating on Al showed superior wear resistance compared to AlN coating.

In this research iron nitride coating was deposited on Al substrate using active screen plasma nitriding technique. Samples were plasma nitrided in an iron cage, at 550 °C, gas mixture of N2:H2% = 3:1 for 2.5, 5 and 7.5 h. The coatings were characterized using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Corrosion resistance of samples was investigated using polarization technique. Mechanical properties including nano hardness, wear performance and adhesion strength were also studied. Results showed that the coatings were mainly consist of Fe2-3N particles whose size increased by nitriding at longer time periods. Moreover, it was found that at longer coating times such as 7.5 h, the nitrogen atoms diffused into the Al substrate and formed a thin layer of AlN under the Fe2-3N coating. Formation of the AlN coating increased the corrosion resistance of the coated sample, but it was still poor compared to uncoated Al. On the contrary, mechanical features of coated Al improved by increasing the nitriding time. Finally, the mechanical characteristics of the deposited coatings were compared to those of conventionally plasma nitrided Al. Despite the higher hardness of the AlN coating, Fe2-3N showed superior scratch hardness and wear resistance.